Ion-Selective Optical Sensors with Internal Reference: on the Quest for Prolonged Functionality and Calibration-Free Measurements

As with most chemical sensors, the proliferation of optodes in analytical routine is hampered by the need for periodic sensor calibration. This fundamental limitation arises from a number of chemical and technological factors (e.g., instability of the illumination source, changes in the composition of the sensor phase, etc.), leading to deviation of the optodes signal in parallel measurements and over time. The development of new approaches for calibration-free determination of analytes with optodes remains a pertinent challenge in the field of chemical sensors. The presented study is aimed at the development and validation of ion-selective optical sensor arrays that do not require calibration. These arrays incorporate an internal optical signal reference composed of polymeric optodes with suppressed exchange capacity. The applicability of such optodes as optical signal standards is demonstrated in a broad range of pH and background electrolyte concentrations: from pH 5 to 9 in solutions of cations and lipophilic anions with concentrations reaching 0.1 M. This experimental observation is supported by theoretical analyses performed by numerical simulation. The study identifies the effect of coextraction and ion exchange on the range of the optical signal stability. Furthermore, it is shown both theoretically and experimentally that by modulating the acidity of the lipophilic indicator in the standard optode, the operational range of the internal reference can be adjusted to the desired pH or electrolyte concentrations. In addition, the validation results demonstrate the feasibility of determining Na⁺ concentration in the sample using internal standards with fair accuracy.